Our success in suggesting an explanation for Geographos has led us to consider
the next most elongated asteroid, S-class asteroid 433 Eros, the target of the
NEAR mission. Eros has many of the same distinguishing characteristics as
Geographos (and our B- and M-class remnant rubble piles). Visual and radar
observations taken during a 0.15 AU pass near Earth in 1975 report that Eros
has a short rotation period (5.27 hours) and a highly elongated shape (
36
x 15 x 13 km;
2.77 x 1.2 x 1.0, normalized; ellipticity
)
([Zellner 1976,McFadden et al., 1989,Mitchell et al., 1998]). Both values are
comparable to those recorded for Geographos and with 15% (30 out of 195) of
our S-, B-, and M-class disruption cases.

Even more intriguing, however, is Eros's pole-on silhouette, which, after
modeling the older Goldstone radar data, looks something like a kidney bean
(Fig. 5) ([Mitchell et al., 1998]). One must be careful not to overinterpret this
shape, since it is based on data that has a signal to noise ratio of
while the shape has been ``fit'' to a reference ellipsoid which can eliminate
discriminating features. In fact, the concave side of the ``kidney bean''
shape may not be a single concavity, but several adjacent ones. Still, we
believe it plausible that Eros's arched back and tapered ends are analogous to
similar features on Geographos, themselves produced by spiral deformations
associated with tidal forces. Images from the NEAR spacecraft should readily
resolve this issue.

fig5

The NEAR spacecraft will offer several additional ways to test our hypothesis.
Regardless of whether Eros is covered by regolith or bare rock, spectroscopic
measurements will suggest a surface composition which can be directly compared
to terrestrial rock samples. If the densities of these samples are
substantially larger than Eros' bulk density, we can infer that Eros is
probably a rubble pile. While observations of large craters would support the
rubble pile scenario, too many would weigh against the tidal disruption
scenario; global landslides caused by a relatively recent tidal disruption
event should modify or bury craters. For this reason, we expect most tidally
distorted objects to have relatively young and spectroscopically uniform
surfaces. As we will describe below, however, the unknown dynamical history of
Eros makes any prediction problematic. Landslides also sort debris as it goes
downhill; high resolution images near the ends of Eros may not only show
cusp-like features but a prevalence of small fragments. An estimate of the
spatial distribution of block sizes inside Eros may come from NEAR's gravity
field maps. Finally, the results of Bottke and Melosh (1996) and Richardson et
al., (1998) show that asteroids affected by tides may often have small
satellite companions which were torn from the original body. Thus, the
presence of a small moon about Eros would be a strong indication that it had
undergone tidal fission.

A possible problem, dynamically-speaking, is that Eros is currently an Amor
asteroid on a solely Mars-crossing orbit (a = 1.46 AU, e= 0.22, ). Test results show that tidal disruption events occur relatively
infrequently near Mars, since it is a weak perturber (Bottke and Melosh 1996).
Studies of Eros's orbital evolution, however, suggest that it may have been
on a low-inclination, deeply Earth-crossing orbit in the past ([Michel et al., 1998]).
Numerical integrations of Eros-type clones show that secular resonances
and
probably modified Eros's orbital parameters, decreasing
its eccentricity enough to place it out of reach of the Earth, while
increasing Eros's inclination to its current value
([Michel et al., 1996,Michel 1997,Michel et al., 1998]). If true, Eros would have been prone to low
velocity Earth encounters (and tidal disruption) in some past epoch.